As a high-density recording technology for magnetic disk drives, a discrete-track magnetic disk (hereinafter referred to as DTM disk) has been developed to improve the magnetic recording characteristics by forming grooves or spaces between plural tracks concentrically formed on the disk, in order to magnetically separate the tracks from each other. In this DTM disk, the tracks are formed on the disk during the manufacturing process, so that it is inevitable that displacement, or eccentricity, occurs between the center of the tracks and the rotational center of the spindle due to an alignment error or other random errors in disk formation and attachment to the spindle.
In the DTM disk mounted on a magnetic disk drive for practical use, the eccentricity can be adjusted by servo information that is integrally formed with the tracks in the manufacturing process, or in other ways, to accurately position the magnetic head on a track. On the other hand, particularly in the initial development stage of the DTM disk, it is difficult to apply the servo controlled head positioning system, because of its strict requirements for providing servo information, a position signal demodulating circuit, a servo circuit, and adjustment of such circuits for each trial production. Thus, in most cases of the DTM disk for trial and evaluation use, only plural tracks are concentrically formed on the disk without including the servo information.
When such a DTM disk on which only tracks are formed is used for evaluating the magnetic recording characteristics by fixing the magnetic head at a predetermined position, the magnetic head crosses many tracks during one rotation of the disk due to the influence of the eccentricity. Thus, it is difficult to evaluate in a continuous area.
In order to overcome this problem, it has been proposed, for example as disclosed in Japanese Patent No. 4012057, to provide a method for compensating the position of the magnetic head. In this method, the intersection between the magnetic head and the tracks is observed from the amplitude variation of the read signal of the magnetic head. Then, the amount of eccentricity is calculated by multiplying the number of intersections by the track pitch, and the direction of the maximum interval between the intersections is determined as the eccentricity direction.